Sandwich panels are used extensively in the aircraft and marine industries. These sandwich panels typically are made of fiberglass or similar material formed in a honeycomb structure. The honeycomb structure is then typically sandwiched between outer layers of aluminum panels or the like. These sandwich panels may be used as interior panels of the aircraft. In order to anchor objects to the sandwich panel, inset panel fasteners are typically used.
Many types of mounting devices have been developed for holding one or more objects to a support surface. For example, inset-type fasteners are generally anchored into a structure and include an engagement portion, such as the male or female threads of a screw, for securing objects to the structure. Inset fasteners are particularly useful when a strong connection between the structure and object cannot be readily obtained through use of a surface mounted structure. Most inset panel fasteners include a cylindrical barrel having a central bore. The central bore typically includes female threads functioning as a fastener element. Alternatively, some inset panel fasteners include a floating fastener element in the form of a nut which also includes female threads which is floatingly positioned within the bore. A flange is typically arranged at the base of these floating nuts. In addition, the inset panel fastener may be coupled to a flexible tab intended to stick to and hold the insert flush within a sandwich panel and to prevent adhesive from leaking onto the exterior of the panel.
To anchor the inset panel fastener in place, a cavity is typically formed directly through the outer layer (e.g., an aluminum layer) into the honeycomb core using a drill, for example. The inset panel fastener may then be positioned within the cavity and an adhesive or other binding material may be injected into the cavity to secure the fastener to the honeycomb core. Alternatively, adhesive may be injected into the cavity of the panel first and the inset fastener may then be pressed into the cavity of the sandwich panel.
The tab typically has pressure sensitive adhesive to removably adhere to the skin of the sandwich panel. This tab adhesive may be weak and fail due to surface irregularity or contaminants on the skin. The tab may be too flexible, permitting the inset panel fastener to sit above or below the surface of the panel or at an angle within the cavity of the sandwich panel. Further, due to the shape of the insert and installation techniques, if the inset panel fastener is positioned too deep within the cavity of the panel, mark-off may show through on a decorative side of the sandwich panel creating a visual defect. For example, the flat bottom and cylindrical shape of the inset panel fastener may act like a piston during installation forcing itself and/or the adhesive against the bottom of the panel cavity. In addition, during adhesive injection, the adhesive may overflow, spilling over the tab and onto the sandwich panel surface. Still further, after the adhesive has set, a scraper may be required to separate the tabs from the inset panel fastener and sandwich panel.
In addition, the floating fastener element typically has a thread locking design that may create problems with thread engagement and galling of the threads. The thread locking design may also lead to stress concentration that may result in cracking of the nut when a screw element is introduced into the nut.